--enable-werror-always fails function.o for any non-pic target

[DJ Delorie]

> >   if (pic_offset_table_rtx)
> > 	add_to_hard_reg_set (&set_up_by_prologue, Pmode,
> > 			     PIC_OFFSET_TABLE_REGNUM);
>
> But if PIC_OFFSET_TABLE_REGNUM == INVALID_REGNUM, then
> pic_offset_table_rtx should be NULL_RTX.

It is, but is gcc smart enough to know that causal relationship?